mlr3 0.1.6

• We have published an article about mlr3 in the Journal of Open Source
  Software: https://joss.theoj.org/papers/10.21105/joss.01903.
  See citation("mlr3") for the citation info.
• New method Learner$reset().
• New method BenchmarkResult$filter().
• Learners returned by BenchmarkResult$learners are reset to encourage the
  safer alternative BenchmarkResult$score() to access trained models.
• Fix ordering of levels in PredictionClassif$set_threshold() (triggered an
  assertion).